The present invention relates generally to medical devices and particularly to a stent assembly with graft layers disposed along luminal and abluminal surfaces of a stent.
Although stent-graft assemblies may be used to treat a number of medical conditions, one common use of stent-graft assemblies relates to the treatment of aneurysms. As those in the art well know, an aneurysm is an abnormal widening or ballooning of a portion of an artery. Generally, this condition is caused by a weakness in the blood vessel wall. High blood pressure and atherosclerotic disease may also contribute to the formation of aneurysms. It is possible for aneurysms to form in blood vessels throughout the vasculature. However, common types of aneurysms include aortic aneurysms, cerebral aneurysms, popliteal artery aneurysms, mesenteric artery aneurysms, and splenic artery aneurysms. If not treated, an aneurysm may eventually rupture, resulting in internal hemorrhaging. In many cases, the internal bleeding is so massive that a patient can die within minutes of an aneurysm rupture. For example, in the case of aortic aneurysms, the survival rate after a rupture can be as low as 20%.
Traditionally, aneurysms have been treated with surgery. For example, in the case of an abdominal aortic aneurysm, the abdomen is opened surgically and the widened section of the aorta is removed. The remaining ends of the aorta are then surgically reconnected. In certain situations the surgeon may choose to replace the excised section of the aorta with a graft material such as Dacron, instead of directly reconnecting the two ends of the blood vessel together. In still other situations, the surgeon may put a clip on the blood vessel at the neck of the aneurysm between the aneurysm and the primary passageway of the vessel. The clip then prevents blood flow from the vessel from entering the aneurysm.
An alternative to traditional surgery is endovascular treatment of the blood vessel with a stent-graft. This alternative involves implanting a stent-graft in the blood vessel across the aneurysm using conventional catheter-based placement techniques. The stent-graft treats the aneurysm by sealing the wall of the blood vessel with a generally impermeable graft material. Thus, the aneurysm is sealed off and the blood flow is kept within the primary passageway of the blood vessel. Increasingly, treatments using stent-grafts are becoming preferred since the procedure results in less trauma and a faster recuperation.
Although stent-grafts are frequently used for treating aneurysms, other medical treatments also use stent-grafts and still other uses are being explored. Additional applications for stent-grafts may also be developed in the future. One example of other uses for stent-grafts is the surgical use of stent-grafts as artificial or replacement vessels. In the case of the vascular system, stent-grafts may be used to replace excised sections of diseased arteries with an artificial replacement vessel. Usually, this would be performed surgically by connecting the ends of the stent-graft to the ends of the artery remaining in the patient's body. Thus, in this application, the stent-graft acts like a blood vessel by directing blood flow through the lumen of the stent-graft and preventing blood flow through the walls of the stent-graft.
Stent-grafts may be used in still other applications as well. For example, stent-grafts may be used to treat stenosed arteries or other vascular conditions. Stent-grafts may also be used to treat a variety of non-vascular organs, such as the esophagus, trachea, colon, biliary tract, urinary tract, prostate and the brain.
One type of stent-graft currently known in the art is constructed with a stent disposed between inner and outer layers of graft material. In order to maintain the integrity of the stent-graft assembly during use, the graft layers must be secured to the stent in some manner. Various techniques for securing graft layers to a stent are currently known. However, the known conventional techniques have numerous problems associated therewith, and an improved manner of securing graft layers to a stent is desired.
One technique for securing graft layers to a stent generally involves adhering the graft layers directly to the stent itself. This is normally accomplished by suturing the graft layers to the struts of the stent or some other part of the stent structure. However, this process must be done manually by specialists using special needles and forceps to sew thread through the graft material, around the struts of the stent, and finally knotting the ends of the thread. This is a very labor intensive task that is time consuming and expensive, thus raising the cost of stent-grafts made by this process.
Moreover, stent-grafts made by suturing the graft layers to the stent lose much of the flexibility inherent in the stent itself. This is generally caused by the direct attachment of the graft layers to the stent structure, which forces the entire assembly (i.e., both the graft layers and the stent) to move simultaneously together. As a result, the graft layers restrict the movement of the stent structure. Flexibility of the assembled stent-graft is important for several reasons. For example, radial flexibility is important to allow the stent-graft to be collapsed onto a delivery system while also allowing the stent-graft to expand at the site of implantation. Axial flexibility is also important to enable the stent-graft to bend as it is guided through tortuous pathways to reach the site of implantation. Even after implantation, axial and radial flexibility remain important when the stent-graft is implanted in an area of the body that is expected to experience frequent movement. However, despite the importance of flexibility, stent-grafts that secure the graft layers directly to the stent are relatively inflexible compared to other types of stents.
Another technique that is used for securing graft layers to a stent generally involves encapsulating the stent or a portion thereof with an inner and an outer layer of graft material. In this type of stent-graft, the two layers of graft material are adhered to each other through open areas in the stent structure. Some additional bounding may also occur between each graft layer and the stent structure itself. Typically, the inner and outer graft layers are adhered by heating the graft layers or with adhesives. However, this type of stent-graft also lacks flexibility as described above. This is due in general to the encapsulated construction of these stent-grafts. In particular, the areas in which the two graft layers are attached abut against the structure of the stent, thereby forcing the graft layers to move together with the stent. This causes the graft layers to restrict the movement of the stent structure. Thus, even when the graft layers are not directly secured to the stent as described, the graft layers are still unable to move independently of the stent.
Accordingly, it is apparent to the inventor that a stent-graft is desired with improved flexibility which allows the graft layers of a stent-graft to move relative to the stent. A solution to these and other problems is described more fully below.